There are several ways to tell if your melons are ripe beyond thumping it to hear the hollow sound. The tendril nearest the melon should have turned brown and become dry.  Another way is look at where the melon touches the ground, there should be a yellow spot. Finally and this seems counter intuitive, when the melons take on a a dull look and give up the glossy skin.  Melons should be grown in a warm environment in rich soil.